When a picture is to be taken against the sun or some other strong light source, tubular light shields of cylindrical or frustoconical shape are frequently used to prevent the direct incidence of rays from that source upon the front lens of the objective. The rim of such a light shield should come as close as possible to the boundary of the field of view of the objective without, of course, cutting off any useful light rays from the object to be photographed.
A photometric cell, serving either to indicate the degree of illumination to the user or to control directly the diaphragm stop, is generally positioned behind a window lying adjacent the objective at the front end of a lens mount. A tube designed to provide maximum protection against incident sunlight would therefore cut across the path of light rays passing from the object to the photocell unless the end of that tube proximal to the lens mount is large enough to encompass both the objective and the photocell window. A tube of such large base diameter, however, must also be of considerable axial height in order to approach the boundary of the field of view at its distal end. Such a light shield is relatively heavy and unwieldy.